1. Field of the Invention
The present invention relates generally to an electrode system intended for proximal connection to a cardioverter/defibrillator, and for distal placement in the heart region for delivering electrical energy from the cardioverter/defibrillator to the heart, for effectively terminating an arrhythmia in the heart. More specifically, the invention relates to such an electrode system having three electrodes, at least two of which are intravascular electrodes.
2. Description of the Prior Art
Defibrillation/cardioversion systems ("cardioversion" in this context referring to defibrillation using lower energy; the generic designation "defibrillation" will henceforth be used below) are known employ a three-electrode system, with two of the electrodes being intravascular electrodes. One of the intravascular electrodes is normally placed in the right ventricle and the other is placed either in the superior vena cava or, less commonly, in the inferior vena cava or in the coronary sinus and its continuation (the great cardiac vein) along the base of the heart. The third electrode in known systems is a subcutaneous electrode with a large area, i.e., a patch electrode. For the purpose of achieving efficient energy utilization of the energy stored in the heart and a good distribution of current in the heart, while simultaneously avoiding major surgery such as opening of the thorax, the subcutaneous patch electrode is usually placed in the vicinity of the left ventricle, between the thorax and the skin.
Electrode systems with the above-described electrode locations are described in a number patents, including U.S. Pat. No. 4,708,145. This patent discloses intravascular electrodes with a common electrode cable for a plurality of electrodes in the right ventricle and superior vena cava. It is also known to provide separate electrode cables for different electrodes, as well as a separate sensing electrode for detecting cardiac events, as described in U.S. Pat. No. 4,727,877, European Application 0 373 953, corresponding to U.S. Pat. No. 5,014,686 and U.S. Pat. No. 5,044,375.
A common feature of prior art electrode systems is the adaptation of one of the electrodes for placement in the right ventricle, and placement of that electrode in the right ventricle. Such an endocardiac electrode imposes strain on the heart in the form of irritation of the musculature of the heart wall and the valves between the atrium and ventricle, as discussed in European Application 92 104 098, corresponding to U.S. Pat. No. 5,235,977. There is also an associated increased risk of blood formation. Therefore, an electrode system is disclosed in European Application 92 104 098 wherein the endocardiac electrode is avoided, and is replaced by an electrode in the inferior vena cava.